The measurement and counting of neutrons is a highly significant problem. Neutrons are for example emitted by special nuclear materials like reactor and/or weapons grade plutonium whereas such materials often do not emit other radiation which can penetrate thick shieldings. Therefore, neutron detection may provide indications or even evidence for the presence of such nuclear materials.
According to ANSI standards, handheld devices for nuclide identification, which are for example used at boarder crossings, are required to comprise a neutron detector. Measurement of neutrons is further of importance in dosimetry, performed in laboratories and nuclear facilities.
As neutrons do not ionize matter, all known neutron detectors are based on reactions between a neutron and another nuclide, thereby generating secondary charged particles or γ-radiation. Therefore the standard detection process typically consists of two steps: neutron conversion, that is the generation of secondary radiation in a converter medium, for example by neutrons scattering or neutron induced reactions like nuclear fission, direct nuclear reactions, neutron capture followed by γ- or charged particle emissions or the like. Afterwards, this secondary radiation is measured with usual nuclear radiation detectors in a ‘detection medium’.
In order to detect neutrons, both processes can be spatially separated, that is converter and detection media are different, or not, namely when the converter medium is the detection medium at the same time.
The detection medium is necessarily sensitive to ionizing radiation. Therefore, if neutrons must be detected in mixed fields, for example in an environment with elevated γ-radiation, the discrimination of neutron against other signals is of major importance. Therefore, neutron detectors in handheld isotope identification devices must not detect γ-radiation from any radionuclide which may be present. Even in the case of strong γ-source, the γ-radiation must not generate false neutron alarms.
The most common thermal neutron detector applied almost wherever robustness is an issue, is a proportional counter filled with 3He gas. 3He is at the same time the converter and the detection medium.
A big disadvantage of such kind of detectors is the neutron detection efficiency. This is approximately proportional to the product of volume and gas pressure. Therefore, such a detector either has to have a large volume or the gas has to be stored under high pressure. As a consequence, the detector can either not be utilized in handheld identification devices because of its large volume or the transportability of such a handheld device is limited, as high pressure devices may be subject to transport regulations like in airplane transportation.
As a matter of principle, neutron detectors with solid converter media are more appropriate when considering the detection efficiency per volume, at the same time not encountering any problems with high pressure gas devices. Such solid media detectors are often scintillator crystals, comprising lithium (6Li), cadmium (Cd), Boron (10B) or other neutron converters. Such a scintillator crystal is called “neutron scintillator”. Such a neutron scintillator is for example 6LiI(Eu), as described in Knoll, Radiation Detection and Measurement, 3rd Edition 2000, page 517.
In such a crystal, the 6Li captures the thermal neutron, thereby generating a tritium (3H) ion and an α-particle with a combined energy of roughly 4.8 MeV. Due to the relatively low quenching of this 6LiI(Eu) crystal, the light signals following neutrons capture correspond to signals which would be generated by γ-radiation with an energy of more than 3 MeV.
As all relevant radionuclides do not emit γ-radiation with such high energies, the light signals, being emitted from the 6LiI(Eu) scintillator crystal can be separated by energy discrimination.
The disadvantage of this prior art is that those detectors have to be used in combination with photomultipliers as light detectors. Other light detectors, namely semiconductor based photo detectors, are sensitive for γ-radiation also, thus acting as γ-detectors themselves generating a much larger signal per unit of deposited γ-energy as in comparison with neutron hits in the scintillator. This is due to the fact that the γ-energy deposed in the scintillator is first translated in a light pulse, only within in a second step generating photo electrons in the semiconductor based photo detector.
As a consequence, a pulse height analysis may be able to distinguish the neutron capture signals from the γ-hits in the scintillator, but not between neutron capture signals in the scintillator from γ- or x-ray hits with much lower energy directly deposed in the semiconductor based photo detector.
Therefore, such a detector could only be used for the detection of neutrons in a surrounding, where more or less no γ-radiation is present. As such surroundings are, at least outside a laboratory, not the surroundings which are of some practical relevance, this is not an option.